One of the problems posed with crown or ring type brakes is how to prevent uneven wear of the friction linings of the brake pads. Generally, such oblique wear of the friction linings is caused by a rocking moment which at least one of the brake shoes is subjected to in the course of operation.
In a crown or ring brake the radially inner brake pad which is adapted to engage the inner surface of the cylindrical brake member is subjected to this rocking moment. During braking the inner brake shoe is subjected to a drag force in the direction of rotation due to its friction contact with the cylindrical brake member and also to a reaction force owing to its being circumferentially supported by the fixed support either directly or indirectly through the caliper at the trailing end relative to the direction of rotation of the brake member.
Owing to the inner pad being convex, the resulting forces produce a rocking moment about an axis perpendicular to the braking axis and parallel to the axis of the cylindrical brake member, which rocking moment causes the circumferentially leading end to bear more against the brake member.
In operation it is found that the wear of the friction pad of the inner brake pad is greater at the circumferentially leading end than the circumferentially trailing end.
Theoretically owing to its being concave, the outer brake pad, which is adapted to cooperate with the outer surface of the brake member, is subjected to no substantial rocking moment. In practice, however, it has been observed that the wear of the friction lining of the outer brake pad is greater at the trailing end than at the leading end. The reason for this is that, in operation, on account of the clearance with respect to the fixed support, for good sliding relative thereto without a risk of jamming, fretting or corrosion whatever the thermal expansion relative to the fixed support, and the inevitable increase in clearance due to the opening of the fixed support caused by braking torque in operation, the caliper tends to follow within the limits of clearance defined above, the tilting of the inner brake pad attendant to oblique wear of the tendency for greater wear at the leading edge, and therefore causes the outer brake pad to bear more through its trailing end than its leading end.
To illustrate the above it is possible to indicate by way of example the angular inclination of the caliper relative to the braking axis may reach an angle of the order of 1.degree.. It follows that the greater wear of the outer brake pad is more than one millimeter, or overall 20% of the thickness of the brake lining material at the trailing end of the outer brake pad.
Thus, in operation, like the inner brake pad, the outer brake pad has a tendency to tilt with respect to the brake member.
Other than the resulting uneven wear of the brake pads, which reduces the service life thereof, the tilting of the brake pads is a source of spurious rubbing between brake operations which untimely wears out the pads.
Such oblique wear of brake pads is also found in disc brakes. In such disc brakes the caliper is fixed and a hydraulic actuating unit is provided at each end thereof, one for controlling the first brake pad and the other for controlling the second brake pad. It has already been proposed to minimize the oblique wear of the brake pads by shifting or offsetting with respect to the central areas of the brake pad the center of area of the brake pad that cooperates with the piston of the corresponding hydraulic actuating unit, which is machined and oriented accordingly.
In practice the center of area of the brake pad is then circumferentially offset with respect to the braking axis of the leading end of the brake pad relative to the direction of rotation of the disc, the bearing surface against which the corresponding hydraulic actuating unit acts being truncated in consequence.
Mere transposition to a floating operating member does not provide a device which gives satisfactory results. For example, with a cylindrical brake member this arrangement produces accelerated wear of the outer brake pad, the angle of inclination of the caliper relative to the braking force axis is accentuated correspondingly.
In French printed patent application No. 2,176,357 which discloses a disc brake with a floating caliper a circumferential offset is provided between the piston of the hydraulic actuating unit and the corresponding (inner) brake pad, and the other (outer) brake pad on the bearing surface side of the caliper.
In practice it is found that such an arrangement is ineffective as a not insubstantial oblique wear of the brake pads has been observed. The reason may be due to the effects of the circumferential offsetting in such an arrangement which may counteract each other.